Interstellar Ca II and Na I line profiles towards halo OB stars

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Abstract
High-resolution Ca II and Na I interstellar line profiles towards several halo OB stars are presented. The profiles have been corrected where necessary for the presence of stellar features and analysed using multi-component models to derive information on the radial velocities, internal velocity dispersions and column densities of individual interstellar clouds. A method is described for estimating peculiar velocities for the clouds, and a significant trend of decreasing R = N(Na I)/N(Ca II) with increasing peculiar velocity is found. The ratio,R, is also shown to be generally smaller in the halo than in the plane, implying that many of the halo clouds may have peculiar velocities. The data indicate that there are both few clouds and a small Ca II density beyond |z| = 2 kpc.

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